1. Field of Invention
This invention relates to a drift tube apparatus and more specifically to a drift tube apparatus which uses electric fields to generate dynamic ion barriers in the drift tube, also referred to as ion storage “wells.” The invention also includes a method for manipulating the voltage gradients that are generated by the electric fields in a drift tube to achieve optimum dynamic collection of ions and the maximum sensitivity to detect these ions. The apparatus and methods described here have practical applications in the field of smoke detectors and ion mobility spectrometry.
2. Background
Drift tubes for the characterizing of gas phase ions date from the early 1900s and descriptions of numerous advanced designs and methods were available by the middle to late 1930s. The large number of experimental configurations that resulted eventually converged on a linear potential gradient that causes a linear ion drift direction in the drift region. Either positive or negative ions can be detected depending on the polarity of the potential gradient that is used. In a drift tube, ions are injected into one end of the drift tube, and drift to the other end where they impinge upon a plate called a detector. A high gain current amplifier connected to the detector provides an electrical output signal. Methods to inject ions into the drift region are based upon electric fields that were transverse to ion drift. Two designs have been popular for creating the transverse fields, namely the Bradbury-Nielson and the Tyndale-Powell designs.
In the Bradbury-Nielson ion shutter design, an electric field is created on adjacent parallel wires or “shutters” which contain alternate potentials. Fields between adjacent wires are usually 3 to 6 times that of the fields responsible for ion drift and penetration of ions through the shutter fields is effectively blocked when this potential difference is applied to the wires. Ion injection is accomplished by eliminating the transverse field by bringing wires to a common potential; ions are drawn through the shutter to drift under the influence of the field gradient along the whole drift tube. In a second approach for ion injection, fields are established between a set of parallel wire gauzes off-set and insulated by a few millimeters; this is the Tyndale-Powell design and was popular in several research laboratories.
Later shutter designs employed in drift tubes were designed so that the wires in the shutter were co-planar and separated by a distance of approximately 0.01 mm with fields of approximately 600 V/cm between wires in contrast to the drift tube field of 300 to 450 V/cm. Currently a hybrid of the prior designs has parallel wires as in the Bradbury Nielson ion shutter which are arranged in separate planes as in the Tyndale-Powell design. In all these designs, the ion shutter comprises mechanically fragile components which complicates the design and manufacture of drift tubes. Nonetheless, the ion shutter is widely used in military and explosive sensing commercial drift tubes and is the subject of contemporary refinements. The only viable alternative to the ion shutter to date has been pulsed photon sources as illustrated by laser ionization.
In the 1980s and 1990s, ion movement was studied at ambient pressure using variations in the duration and location of electric field gradients in drift tubes that resembled mobility spectrometers. In particular, William C. Blanchard designed and demonstrated a planar drift tube where ions could be isolated based upon mobility by repeated passes through a drift region, this work being embodied in U.S. Pat. No. 4,855,595.
This demonstrated that ions at ambient pressure could be manipulated by varying electric fields established by ordinary drift ring electrodes. Subsequently, ion injection into a drift tube was shown using varying electric fields instead of a shutter grid and the concept of collecting ions between two higher electric field gradients suggested an ion barrier or ion storage “well” at ambient pressure. Since most of the ions being created were collected instead of being neutralized as was the case with a shutter grid, a much lower Curie radioactive source could be used. A 0.9uCi241Am source was used with this ion barrier drift tube, instead of the 10 milliCi of 63Ni normally used with shutter grid drift tube designs.
The foregoing reflects the state of the art of which the inventor is aware, and is tendered with a view toward discharging the inventors' acknowledged duty of candor, which may be pertinent to the patentability of the present invention. It is respectfully stipulated, however, that the foregoing discussion does not teach or render obvious, singly or when considered in combination, the inventor's claimed invention.